Mechanism of Initiation of Polymerization by Ferric Acetylacetonate

Results on the autooxidative decomposition of the transition metal B-diketonates (1-3) suggested the possibility of these chelates acting as initiators for the radical polymerization of vinyl monomers. Research in this field soon revealed the importance of the central metal atom as well as the ligand structure (4-10). Of a series of metal acetylacetonate~-Mn(Acac)~, M n ( A ~ a c ) ~ , Fe( A ~ a c ) ~ , Co(Acac)2, N i ( A ~ a c ) ~ etc. Mn(Acac), was found to be the most effective initiator; its efficiency was comparable to that of tertiary butyl hydroperoxide. The striking influence of the ligand in determining the activity of the chelate was brought about through the studies carried out by Bamford et al. (7) on M n ( A ~ a c ) ~ , Mn(p-p'dichloro DBM)3, Mr~(FAcac)~, and Mn(DBM)3. A monomer order of higher than unity is a widely observed and naturally accepted feature of the chelate-initiated polymerization systems. The order with respect to the chelate is almost always close to 0.5. Bamford suggested, in agreement with Arnett and Mendelsohn (1-3), that the initial initiation step involves the scission of the ligand as a free radical and the reduction of the metal as in the following equation: